Sun exposure is one of the important factors that cause skin aging. Intense ultraviolet radiation may cause skin cancer. Studies have shown that ultraviolet rays in the UVB band (290-320 nm) can pass through the stratum corneum and epidermis, causing skin erythema, leading to DNA damage. Ultraviolet rays in the UVA band (320-400 nm) can reach the dermis and is an important factor in triggering melanoma. Therefore, effective sun protection is necessary. Sun-screening agents can effectively absorb or scatter ultraviolet rays in the UVB and UVA bands of sunlight, and are generally classified into two major categories: physical and chemical. Physical sun-screening agents such as titanium dioxide and zinc oxide and other inorganic particles can reflect or scatter ultraviolet radiation, but have poor affinity with the skin, easily accumulate on the hair follicle, and have poor comfort, which make them have certain limitations in practical applications. Therefore, the active ingredients in sun-screening skincare products are mainly chemical sun-screening agents.
The sun-screening skincare products based on the chemical sun-screening agents have more delicate texture, a relatively wide sun-screening spectrum and a relatively strong sun-screening ability, and are thus generally welcomed by consumers. However, the chemical sun-screening agent itself can absorb ultraviolet rays, has photochemical activity or physical activity, and undergoes photodegradation after absorbing ultraviolet rays, resulting in a decrease in the ability of the sun-screening agent to absorb ultraviolet rays, a poor sun-screening effect for a long time, and also an increase in irritation and allergy of the skin caused by the degradation products.
In addition, the chemical sun-screening agents are generally micromolecular compounds that can penetrate skin cells and trigger DNA variation, and are easy to have their molecular chains broken under illumination to produce highly active free radicals that can damage biological macromolecules and various cells and then lead to skin aging or skin cancer.
In order to solve the above problems, researchers at home and abroad have developed a variety of substrate systems such as liposomes and gels to coat the chemical sun-screening agents. The stability of the coated chemical sun-screening agents is greatly improved, and the chemical sun-screening agents can be isolated from the skin, reducing the toxic side effects on the skin. Liu Shuangxi and Song Chunjin developed a preparation method of a chemical sun-screening agent coated with chitosan gel as a wall material. First, gelatin, the chemical sun-screening agent and an emulsifier are emulsified at a high speed, and then a mixed solution of chitosan and acetic acid is added to the primary emulsion, followed by re-agglomeration with calcium chloride and curing with glutaraldehyde as a crosslinking agent to form a chemical sun-screening agent embedded body (CN 1709219 [P]). Zhang Wanping and Niu Wenxia effectively coated the octyl methoxycinnamate chemical sun-screening agent with solid lipid nanoparticles and inhibited the penetration of the chemical sun-screening agent into the skin by controlling the particle size of the coating carrier (Zhang Wanping, Niu Wenxia, Preparation and characterization of solid lipid nanoparticles loaded octyl methoxycinnamate [J], CIESC Journal, 2011, 62(10): 2965-2968). Deng et al. prepared polylactic acid-polyglycerol by multi-step synthesis for embedding a chemical sun-screening agent, and further hydroformylated polyglycerol to prepare polymer/chemical sun-screening agent microcapsules with bioadsorption properties (Nature Materials, 2015, 14: 1278-1285). Whether it is chitosan and liposome, or polylactic acid-polyglycerol by multi-step synthesis, although the chemical sun-screening agent can be embedded by chemical crosslinking, it is necessary to add various additives such as an emulsifier and a crosslinking agent. The addition of these additives not only increases the cost of preparation and separation, but also causes the hurt of the additive residue to the human body and skin. Besides, the embedding and retention properties of the above wall materials for the chemical sun-screening agent are yet to be further evaluated.
Lignin is a natural polymer compound widely present in the cell wall of plants. Many functional groups such as a benzene ring, a double bond, a carbonyl group and a phenolic hydroxyl group in the molecule of lignin determine that lignin can not only absorb ultraviolet rays, but also have the function of scavenging free radicals, and is thus a natural UV absorber and antioxidant. With lignin having excellent UV and oxidation resistance and good dispersibility, Qian et al. mixed alkali lignin with a hand cream without a sun-screening effect; when the lignin content was 10%, the sun-screening index (SPF value) of the mixed cream reached 5.72; when the alkali lignin was mixed with a commercially available sun-screening cream with an SPF of 15 and the amount of the alkali lignin was 2%, the SPF value of the mixed cream reached 35.32; and when the amount of the alkali lignin was 10%, the SPF value of the mixed cream even reached 89.58. These indicate that lignin not only has broad-spectrum UV protective properties, but also synergizes with chemical sun-screening agents in sun-screening creams (Green Chemistry, 2015, 17: 320-324).
Studies have shown that even industrial lignin is basically not cytotoxic and has good physiological compatibility after purification. These studies have largely eliminated the safety concerns of lignin in the cosmetic skincare field and even in the medical field (Bioresources Technology, 2008, 99(14): 6683-6687). Whether in terms of theory and experimentation, or in terms of ecology and safety, lignin can be developed into an anti-UV natural polymer wall material used to embed chemical sun-screening agents to prepare composite microcapsules, and is used in the field of sun-screening skincare.